In different kinds of small battery-operated electronic devices, such as calculators, electronic games, info displays, GPS devices (Global Positioning System), remote controls, portable CD (Compact Disc), MD (Minidisc), DVD (Digital Versatile Disc) and other players, digital cameras and particularly in mobile telephones, liquid crystal displays have commonly been used for presenting different types of data, among other things, a telephone number. Because a liquid crystal display is not by nature self-illuminating, it must be illuminated at least in the dark so that a user, even in the dark, can see the text and/or graphics presented by it. Older mobile phones mainly used their display for presenting text, less for displaying images or figures. Most commonly, a monochrome, e.g. black-and-white display has been used in them. Especially in small battery-operated mobile phones, LEDs (Light Emitting Diode) have been used as background display illumination. These are indeed very suitable for the background illumination of small displays, being components that take little space and consume little energy. This being the case, LEDs have been selected for background illumination such that their colour is pleasant to the user and the minimum voltage required for their operation is sufficiently low so that the voltage of the mobile phone's battery is sufficient for their use. For example, in a Nokia 6110 pocket telephone the display is illuminated by a set of green LEDs.
As technology has developed, it has become possible to reduce the operating voltage of mobile stations and, thus, also the output voltage of batteries. In the Nokia 6110 mobile phone, used as an example, it is possible to use as the battery a lithium battery, which comprises one battery cell. The voltage of this type of battery is approximately 4.1 V when fully charged and when partly discharged, the battery voltage is lower, approximately 3 V, at the lowest. A low voltage improves a mobile station's energy economy but at the same time, it limits the number of background illumination LED alternatives that can be used without special auxiliary equipment, because some commercially available LEDs require a higher operating voltage. This applies particularly to blue and white LEDs. For example, the operating voltage of mobile stations not only changes as the battery voltage drops, but it also otherwise pulsates, e.g. due to the substantial and pulsating power consumption of the mobile stations's transmitter, which can easily be seen as alternating illumination intensity.
However, illumination is unnecessary if no additional illumination is required, e.g. in a light operating environment. In that case, illumination consumes energy unnecessarily and shortens the battery set's operating time. In some devices, additional illumination is switched on and off according to the brightness of the environment. On the other hand, in total darkness additional illumination should not be very bright so that it does not dazzle the user. It would indeed be an advantage if the brightness of illumination could be adjusted.
In addition to the transfer of voice, plans have been made to include an image transfer functionality in future mobile stations and it should even be possible to present display colour images on the display of a mobile station. In that case, coloured background illumination distorts the colours of the display. Therefore, a colour display should most preferably be illuminated by white light. Sources of white light that are reasonably well suitable for the purpose include cold cathode fluorescence tubes, white electroluminescence panels, as well as white LEDs. Of these, cold cathode fluorescence tubes would require complex operating electronics, the tubes would break easily and the solution would be large and expensive. The problem with white electroluminescence panels is their short service life and the low quality spectrum of light. White LEDs, on the other hand, are based on blue LEDs and their use would otherwise be a good solution but they typically require a minimum of 3.2 V-3.5 V forward voltage in order to operate. However, this exceeds the minimum supply voltage value of a mobile station operated by a single cell lithium battery and requires the use of an energy converter, e.g. a switching circuit. A switching circuit adds to the price and complexity of a solution based on white LEDs and reduces the efficiency by also converting electrical energy into heat. The efficiency of a switching circuit suffers particularly when raising the voltage from a low value to one only slightly higher, for example, from three to four volts, in which case the efficiency may remain at approximately 75%, depending on the size and type of the switching circuit. The use of a switching circuit also requires a suitable oscillator circuit. In addition to the price, the disadvantage of complex structures is the space that must be reserved for them, i.e. they increase the size of a mobile station and add to its weight. Switching mode power supplies suitable for the illumination of a small battery-operated device are, among others, flyback and step up converters. In the following, special methods and equipment for LED illumination, known from prior art will be presented.
U.S. Pat. No. 4,673,865 teaches the feeding of power to a LED through a capacitive circuit by a step up, i.e. booster converter, in principle. The converter comprises both a diode and a "filter capacitor". U.S. Pat. No. 4,864, 193 presents a circuit for using a LED. A device according to the publication reduces distortion in the LED's supply voltage and, thus, improves the operation of the LED in a communication application. The power supply of a set of LEDs, presented by U.S. Pat. No. 5,661,645, comprises an independent converter capable of operating even without load, which has a diode and a large capacitor. U.S. Pat. No. 5,736,881 describes a resonant converter intended for use as an LED power supply. In addition to the LEDs, the converter also comprises a separate diode. The abstract of the invention explains that the invention is a significant improvement in the field of power sources that produce a high and steady current. The invention is not particularly suitable for the purpose of illuminating a portable device that demands a low current, wherein even the steadiness of the current is not critical. U.S. Pat. No. 5,739,639 describes an emergency power source as the power supply of a set of LEDs to be used, e.g. for the illumination of an emergency sign. However, the device is complex in structure and it comprises both capacitors and diodes, which cause power losses. The December 1998 issue of Electronics World magazine presents a circuit for the use of white LEDs using a minimum output voltage of 1.5 V. The circuit is based on a multivibrator. The circuit is described as enabling the use of LEDs that operate at a forward voltage of 3 V . . . 3.6 V, with a 3-volt battery, the voltage of which at minimum is no less than 1.5 V. The current strength of the power feed of the LEDs used cannot be adjusted.
Common to these LED power supply circuits is that they all have components that cause significant power losses, e.g. a diode and in most cases, they also have a bulky large-valued capacitor.
Patent No. JP 9-74224 presents several different types of circuits with which the supply voltage can be lowered to the level of the forward bias voltage of the LED used. FIG. 1 shows the circuit of a step-down converter, wherein a coil 6 is connected via a semiconductor switch 5 in parallel with a direct voltage source and a LED 2 is further connected in parallel with the coil. In parallel with the direct voltage source, before the switch there is a control circuit 7. The control circuit has an output, which is connected to the switch 5 to control the operation of the switch with the help of a control signal. The control circuit measures the voltage of the voltage source and adjusts the pulse width of the control signal (PWM signal, Pulse Width Modulation) based on it. Thus, the control circuit switches the voltage source in parallel with the coil for pulses having a duration depending on the voltage of the voltage source and again renews the switching at prescribed intervals, whereupon the coil discharges its stored magnetic energy as electricity through the LED and produces light energy at a constant level. The publication also presents a feedback circuit, wherein the current that passes through the coil 6 is measured with the aid of a resistor and the coil's charging circuit is switched off by the switch 5 when the current reaches a specified threshold level. (Timing corresponds to FIG. 2). The circuit is implemented without the diode and capacitor found in an energy converter, but the circuit presupposes that the control circuit 7 must comprise an oscillator in order to produce a pulsating signal, which naturally adds to the price of the circuit and increases power consumption. Neither can the illumination power produced by the circuit be adjusted according to conditions.